A femto base station corresponds to a type of base station, which corresponds to a small sized version of a macro base station that can perform most of the functions of the macro base station, and which may be installed in a cover area of the macro base station or in a shadow area that cannot be covered by the macro base station. The femto base station is equipped with a network configuration that can be independently operated. And, as compared to relay base stations, a remarkably larger number of femto base stations may be installed in downtown areas or indoor areas.
FIG. 1 corresponds to a structural view of a wireless communication system additionally including a femto base station.
As shown in FIG. 1, the wireless communication system additionally including a femto base station includes a femto base station (110), a macro base station, a femto network gateway (hereinafter referred to as “FNG”) (130), an access service network (hereinafter referred to as “ASN”) (140), and a connectivity service network (hereinafter referred to as “CSN”) (150). Herein, the macro base station refers to a general base station that is included in the related art wireless communication system.
The femto base station (110) directly accesses a TCP/IP (transmission control protocol/internet protocol) network and operates independently, just as the macro base station. Herein, it is assumed that the coverage of the femto base station (110) is within range of approximately 0.1˜30 m. And, it is also assumed that one femto base station (110) may accommodate about 10˜20 terminals. Herein, the femto base station (110) may either use the same frequency as that of the macro base station or use a different frequency (in case of Inter FA).
The femto base station (110) is connected to the macro base station through an R1 interface, thereby being capable of receiving a downlink channel of the macro base station. And, the femto base station (110) may transmit a control signal to the macro base station.
The femto base station (110) is capable of covering indoor or shadow areas that cannot be covered by the macro base station and may also support a high data transmission rate. The femto base station (110) may be installed within the macrocell in an overlay format, or the femto base station (110) may also be installed in a non-overlay format in an area that cannot be covered by the macro base station.
The femto base station (110) may be divided into two different types. A first type corresponds to a CSG (closed subscriber group) femto base station, and a second type corresponds to an OSG (open subscriber group) femto base station. The CSG femtocell base station groups terminals that can access the CSG femto base station and assigns CSG ID (identification) to the accessing terminals. Thereafter, the terminals that are assigned with a CSG ID may be differentiated from the terminals that are not assigned with a CSG ID when accessing the CSG femto base station. The OSG femto base station corresponds to a base station that allows the access of all terminals.
The FNG (130) is a gateway that is configured to control the femto base station (110). Herein, the FNG (130) is respectively connected to the ASN (140) and the CSN (150) through an Rx interface and an Ry interface. The femto base station (110) may receive service from the CSN (150) through the FNG (230), and the terminals that are connected to the femto base station (110) may be serviced with functions, such as certification, IMS, and so on, from the FNG (130) or the CSN (150).
The CSN (150) provides the terminals with connections to application services, such as the internet, VoIP, and so on, and also provides the terminal with certification (or authentication) and charging functions. And, the ASN (140) controls the macro base station and manages the connection between the macro base station and the CSN (150).
Meanwhile, the CSG type femto base station may be categorized into 2 different types depending upon the accessibility of a non-member terminal, i.e., a terminal that has not been assigned with a CSG ID.
A CSG-Closed Femto ABS corresponds to a femto base station type that authorizes (or allows) access to member terminals only. Accordingly, a terminal may store CSG-Closed Femto ABS identifiers that allow the access of the corresponding terminal in a White list.
Meanwhile, a CSG-Open Femto ABS provides first priority support to member terminals. And, when surplus support remains, the CSG-Open Femto ABS may also allow access to non-member terminals. However, the service level that is provided to the non-member terminals may be differentiated from the service that is provided to the member terminals.
In the mobile communication systems that presently uses the Femto base stations, it is assumed that the categorization for the CSG type femto base stations is fixed to the two different types that are described above with respect to the access authorization level provided to the non-member terminals. Herein, the distinction for CSG Open/Closed femto base stations may be known from an AAI_SCD (Advanced Air Interface System Configuration Descriptor) through partition information.
Hereinafter, a handover will be described in detail.
A Handover (HO) refers to a shift (or repositioning) of a terminal from a wireless interface of one particular base station to a wireless interface of another base station. Hereinafter, a handover procedure within an IEEE 802.16m system.
Presently, in an IEEE 802.16m, the HO is broadly divided into 3 different procedures: HO decision and Initiation, HO preparation, and HO Execution. Prior to performing (or executing) the above-mentioned 3 procedures, the terminal is required to perform a procedure of gathering information on the neighboring (or surrounding) base stations. Such scanning procedure is similar to the scanning procedure performed in a general IEEE 802.16e network. However, when there exists a section during which the terminal does not communicate with the S-ABS, this section may be used for scanning the neighboring base stations.
During the HO decision and Initiation procedure, the terminal or the base station may initiate the handover procedure. When the terminal initiates the handover procedure, the terminal transmits a handover request message (AAI_HO-REQ) message to the serving base station. At this point, depending upon the decision of the base station, the terminal decides whether or not to perform (or execute) EBB handover.
During the HO preparation procedure, a serving base station (S-ABS) and a target base station (T-ABS) exchange terminal information with one another, and the serving base station (S-ABS) and the target base station (T-ABS) negotiate conditions for the HO execution procedure that will be carried out later on and also negotiate resources (dedicated code, STID, security parameters etc) that are to be allocated to the terminal. And, in some cases, the S-ABS may provide the terminal with multiple T-ABSs. When the terminal initiates the HO, the terminal requests for an HO through an AAI-HO-REQ message. Thereafter, the terminal receives the above-described information from the base station through a handover command (AAI_HO-CMD) message. At this point, an action time and a disconnect time in included in the AAI_HO-CMD message, thereby being transmitted. Herein, the action time refers to a time during which the terminal performs network re-entry, and the disconnect time refers to a time during which the S-ABS stops performing uplink/downlink (DL/UL) resource allocation to the user equipment.
The HO execution procedure corresponds to a time during which a network re-entry to the T-ABS is performed in accordance with the action time. At this point, network re-entry may be performed through a CDMA ranging or immediate ranging request (AAI_RNG-REQ) message. When the terminal performs EBB (Entry Before Brake), the terminal may consistently (or continuously) information to the S-ABS at a pre-determined AI (available interval), and the terminal may perform network re-entry to the target base station at a pre-determined UAI (unavailable interval). When the terminal performs BBE (Break Before Entry), the AI or UAI remains undecided, and the procedure is similar to a Hard Handover process of a general IEEE 802.16e network.
Hereinafter, the above-described handover procedure will be described in more detail with reference to FIG. 2.
FIG. 2 illustrates an exemplary handover procedure in a general IEEE 802.16m system.
Referring to FIG. 2, the handover procedure may be initiated by the serving base station (S201a), or the handover procedure may be initiated by the terminal (S201b). When the handover procedure is initiated by the serving base station, in order to request for a handover to the serving base station, the terminal may perform a process of transmitting a handover request (AAI_HO-REQ) message to the serving base station. However, the serving base station may initiate the handover procedure without having to perform the process of receiving the corresponding message from the terminal.
When the terminal receives the handover command (AAI_HO-CMD) message from the serving base station, the terminal may selectively (or optionally) transmit a handover indication (AAI_HO-IND) message to the serving base station (S202). The terminal may transmit the handover indication message to the serving base station under the following conditions.
1) when the terminal wishes to cancel the handover based upon the conditions decided by the S-ABS
2) when multiple target base stations are included in the AAI_HO-CMD message, and when the terminal select any one of the multiple target base stations
3) when connection is not available to any of the candidate target base stations included in the AAI_HO-CMD message
4) when the connection between the terminal and the serving base station cannot be maintained before the disconnect time expires
Subsequently, the terminal performs network re-entry to the target base station (S203).
If EBB handover is performed, or if a scheduling of the serving base station exists before the disconnect time expires, among the network re-entry process steps, the terminal may perform data exchange with the serving base station (S204).
When the network re-entry is completed, the target base station may notify the completion of the handover process to the serving base station (S205), and the terminal may successfully perform data exchange with the target base station (S206).
During the above-described handover process, the terminal transmits the handover request (AAI_HO-REQ) message to the serving base station, so s to initiate the handover process. And, the serving base station transmits a handover command (AAI_HO-CMD) message to the terminal so as to direct (or order) the handover process. At this point, in case the terminal or the serving base station has failed to successfully transmit or receive the handover associated MAC management message, a problem may occur in the handover process.
For example, when the terminal initiates the handover process, the handover process may begin with the transmission of the AAI_HO-REQ message. And, when the terminal receives the AAI_HO-CMD message from the S-ABS, the handover process may be executed (or performed) in full scale. At this point, in case the AAI_HO-CMD message is lost, and in case the terminal is incapable of receiving the AAI_HO-CMD message, the base station may re-transmit the AAI-HO-CMD message, or the terminal may re-transmit the AAI_HO-REQ message. Thus, the stability in the handover process may be ensured so that the handover process can be performed successfully.
However, the femto cell base station has a narrower coverage as compared to the macro base station. Eventually, in a region where multiple neighboring femto base stations exist, the terminal may be capable of performing (or executing) the handover process frequently. However, in case the handover process is performed each time the terminal is located in such region, a considerable amount of delay time may occur. And, accordingly, a considerable amount of radio resource may be wasted in order to perform communication between the terminal and the corresponding base station for the handover process.